It is well known and has been shown by many investigators in the past that there is a significant amount of wasted energy in the exhaust gases of internal combustion engines. These losses result from the fact that each piston in its associated cylinder reaches the bottom of its power stroke before the hot combustion gases are fully expanded to ambient pressure. In order to recover an appreciable amount of this wasted energy, the exhaust gases must be expanded to a very large volume. The most practical manner of achieving such further expansion is to pass the gases through a turbine. The turbine, in turn, is drivably connected to a turbocharger for the engine and/or to the engine crankshaft.
U.S. Pat. Nos. 2,607,189 and 2,625,006, assigned to the Curtis-Wright Corporation disclose such a combination internal combustion engine and power recovery turbine and the devices described have proved enormously successful in aircraft operation. However, even with the engine turbine combination, it has been found that about forty per cent of the available energy in the gases passed to the turbine is lost before reaching the turbine. Most of the losses are the result of throttling losses (sometimes called "dump losses" or "wire drawing losses") across the exhaust valve flow areas during the valve opening process.
In the U.S. Army's 525 CID VHO six cylinder diesel engine, at rated speed and power, the pressure in the cylinder at the instant that the exhaust valve starts to open (crank angle of 118.degree. from TDC) is 444 inches of mercury absolute. The temperature in the cylinder at that instant is 1827.degree.F. Also at that instant, the pressure in the exhaust stack just downstream of the exhaust valve is about 99 inches of mercury absolute as a result of exhaust from other cylinders. Therefore, just after the exhaust valve starts to open, the high pressure gases in the cylinder start to bleed into the exhaust stack which is at a much lower pressure. There is a significant loss in total pressure and available energy during this opening process.
Ideally, it would be desirable to have the exhaust valves open instantaneously which would eliminate all dump losses. However, in practice this is not possible. In most engines operating at rated speed, the exhaust valves already open as fast as stresses will permit. Accordingly, some other solution to reduce dump losses is needed.
In a publication entitled THE BROWN BOVERI REVIEW, dated November, 1950, pp. 433-439, there is discussed in considerable detail the foregoing problem of lost energy in the exhaust gases prior to reaching a power recovery turbine in an internal combustion engine - turbine combination. In this discussion as well as in the aforementioned patents, it has been the practice to make the exhaust stacks or conduits connecting the exhaust valve flow areas to the turbine of greater internal cross sectional flow area than the actual cross sectional flow area of the exhaust valve when in opened position. The latter mentioned publication goes into substantial detail with respect to dimensioning of these exhaust stacks and concludes that the cross sectional area should not be substantially greater than the fully opened exhaust valve passage, but rather of the order of about 1.20 times the exhaust valve passage. Apparently, the primary considerations in past constructions was to utilize an exhaust stack size greater than the area of the exhaust valve when open in order to reduce the back pressure on the piston during the exhaust stroke to provide maximum reciprocating engine horsepower.